Process for the production of terphenyls



United States Patent 3,207,799 PROCESS FOR THE PRODUCTIUN OF TERPHE'NYL'S Maurice Jaymond, Lyon, France, assignor to Societe Progil, Paris, France, a corporation of France Continuation of application Ser. No. 156,292, Dec. 1, 1961. This application Sept. 22, 1964, Ser. No. 400,301

1 Claim. (Cl. 260-670) This invention relates to a process for the production of terphenyls and, more particularly, to a method for producing terphenyls by the pyrolysis of a gaseous mixture of benzene and diphenyls. This is a continuation of my co-pend-ing patent application Serial Number 156,292, filed December 1, 1961, and now abandoned.

Terphenyls have, in the past, been manufactured merely as a by-product in the manufacture of diphenyl. Considerable work has been done in the manufacture of diphenyl by the pyrolysis of benzene and these processes have been perfected to the point where the by-products are reduced to a minimum. These by-products are especially coke, tars, and higher polyphenyls. Among these polyphenyls are .the terphenyls (sometimes known as triphenyls) mixed with tetraphenyls and other polyphenyls, but these polyphenyls exist in such proportions that their separation .is not economical. On the other hand, it is possible to form the three isomers of terphenyl by the cracking of diphenyl according to the following reaction:

The preparation of terphenyls by such a method is uneconomical, since the diphenyl is a very expensive starting material and the yields of the process are not good. These and other difliculties experienced with the prior art processes have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a process for producing terphenyls in which the consumption of diphenyl is reduced to a minimum.

Another object of this invention is the provision of a method of producing ter-phenyls by the pyrolysis of a gaseous mixture of benzene and diphenyl wherein the terphenyls are obtained with good yields and with very few by-products or impurities, no sulfur or sulfur compound being present in said mixture.

A further object of the present invention is the provision of a process for producing terphenyls independently of the production of diphenyl or any other compound.

Although the novel features which are believed to be characteristic of this invention will be particularly pointed out in the claim appended hereto, the invention itself as to its objects and advantages, the mode of its operation and the manner of its organization, may be better understood by referring to the following description taken in connection with the accompanying drawings forming a part thereof, in which:

FIG. 1 is a schematic view of apparatus for preforming the process incorporating the principles of the present invention,

FIG. 2 is a graph of percentage of diphenyl by weight in the gaseous mixture plotted against the pyrolysis time in second, and

FIG. 3 is a graph in which the percentage of diphenyl by weight in the gaseous mixture is plotted against the pyrolysis temperature in degrees centigrade.

Referring first to FIG. 1 which shows the general concept of the invention, the apparatus, indicated generally by the reference numeral 10, is shown as consisting of an initial mixing chamber 11, a pyrolysis furnace 12, a condensing chamber 13, and a separation chamber 14. An entrance pipe of the mixing chamber 11 is connected to a source of benzene (not shown). An outlet pipe 16 connects the chamber 11 to a heat exchanger 17 which, in the preferred embodiment, is a sinuously-formed tube of stainless steel. The outlet of the heat exchanger 17 is connected to the condensing chamber 13 by a pipe 18. The condensing chamber contains a cooling coil 19 through which cold water or other cooling fluid is passed. The outlet of the condensing chamber 13 is connected by a pipe 21 to the inlet tube to the separation chamber 14. The separation chamber has one outlet for terphenyls to which is attached an outlet pipe 22 leading to a storage means (not shown). Another outlet of the separation chamber 14 is connected by a pipe 23 to another entrance to the mixing chamber 11, this pipe being intended to carry separated dipheny'ls and benzene back to the mixing chamber. The pyrolysis furnace 12 is provided with heating means, such as gas burners 24. The walls of the pyrolysis chamber are formed of refractory material. in the usual manner.

The operation of the process will now be readily understood .in view of the above description. Raw benzene enters the mixing chamber 11 continuously, while diphenyl and unconverted benzene enters through the pipe 23. These constituents are mixed and form a vapor which passes through the pipe .16 into the heat exchanger 17. The gaseous mixture of benzene and diphenyl passes through the heat exchanger 17 and is subjected to the heat in the interior of the pyrolysis chamber 12 produced by the burning of gas in the burners 24. The resulting gas from the pyrolysis reaction passes through the outlet pipe 18 into the condensing chamber 13 where it is subjected to contact with the cooling coil 19 and is changed to the liquid phase. The liquid passes through the pipe 21 into the separation chamber 14 where the terphenyls are passed outwardly through the pipe 22 while the diphenyl and unconverted benzene component passes along the pipe 23 back to the mixing chamber 11 for recycling.

The new process consists in the pyrolysis of a mixture of benzene and diphenyl, under conditions which favor the formation of a high proportion of terphenyls; the pyrolysis is preferably carried out .in a manner so as to produce terphenyl solely from the benzene present in the mixture, according to the reaction:

The three tenphenyls of the molecular formula C H are the isomers where a benzene ring is connected to a diphenyl molecule at the ortho, meta, and para positions.

The process of the invent-ion furnishes particularly good yields of terphenyls from benzene, according to the above reaction, when the reaction mixture contains not only the benzene but a substantial proportion of diphenyl, and the temperature and duration of the pyrolysis are correctly chosen.

FIGURES 2 and 3 indicate how the proportion of terphenyls formed depends upon the amounts of diphenyl in the pyrolysis mixture, at various temperatures and reaction periods. These graphs are representative of the experiments carried out in perfecting the present invention.

FIGURE 2 is a graph of the percentage composition by weight of the reaction mixture after one passage down a heated pyrolysis tube, plotted against the residence period of the pyrolysis vapors in the pyrolysis zone (i.e., the effective period available for reaction).

The curves lettered D, T, and L show the respective percent proportions by weight at the end of the pyrolysis reaction of diphenyl (D), terphenyls (T), and heavier compounds, especially tetra and other polyphenyls (L).

D1, T1, and L1 relates to pyrolysis at 850 C.; D2, T2, and L2 relate to pyrolysis at 785 C.; D3, T3, and L3 relate to pyrolysis at 750 C.

The curve of FIG. 2 show that the best proportions of diphenyls to be employed in each case, is that proportion which does not vary during the pyrolysis. This is precisely the proportion which leads to the best rate of formation of terphenyls solely from the benzene present in the mixture.

Curve D1 shows the proportions of diphenyl which do not change during the pyrolysis at 850 C. At 785 C., following curves D2 and T2, it can be seen that with a residence period of two seconds, it is necessary to use a concentration of 20.5% of diphenyl to obtain a production of 3.5% of terphenyls; if the residence period is 10 seconds, it is necessary to use a concentration of 26.5% of diphenyl and the proportion of terphenyls formed reaches 9%.

Surprisingly, there is a tendency of the curve D to become parallel to the abscissa. In other words, for a given temperature the optimum proportion of diphenyl in the pyrolysis mixture tends towards a constant. Furthermore, the rate of formation of terphenyls increases quite rapidly with the increase in residence time, whilst the divergence between the proportions of terphenyls formed and that of the heavier compounds increases, at least below 850 C.

On FIGURE 3, the ordinate is again that of percent by weight, while along the abscissa pyrolysis temperature is plotted. Two series of curves are shown; D5, T5, and L5, respectively, representing the proportions of diphenyl, terphenyls and heavier compounds as a function of temperature, for a residence period of five seconds; the series D7, T7, and L7 represents a residence period of seven seconds.

Here again it should be noted that the optimum proportions of diphenyl-curves D and D7--tend towards a constant value. Comparison between curves T5 and L5 or T7 and L7 leads to the conclusion that the rate of formation of terphenyls increase with temperature, it being better to operate below a temperature of 850 C., and preferably below 800 C., so as to have the smallest proportions of heavier compounds in the reaction product. Thus, if the temperature does not exceed 750 C., for a residence period of more than seconds, the ratio by weight of polyphenyls to terphenyls produced can fall as low as 0.25.

While the pyrolysis temperature of the benzene can be varied, it is preferably between 680 and 820 C.; these values can represent either a constant temperature along the whole length of the pyrolysis zone, or the average of the temperatures at the extremities of this zone.

The ratio of benzene to diphenyl in the pyrolysis mixture is important; according to the invention, the mixture contains from 5 to 50% by weight of diphenyl. However, it is preferable to have from 10% to 35% of diphenyl in the mixture; a purer product is thereby obtained and the process is more economic since the terphenyl is then formed by consumption solely of the benzene, and not of the more costly diphenyl.

It is particularly recommended to use mixtures of 70% to 80% of benzene with respectively 30% to 20% of diphenyl at mean temperatures of 700 C. to 750 C., with residence periods of 10 to 50 seconds.

The importance of a careful control of the proportion of diphenyl in the vapors to be pyrolyzed is explained by the fact that, outside of the range of optimum proportions, the reaction is accompanied by secondary reactions. Thus, if excess diphenyl is present, part of this is also pyrolyzed to yield terphenyls; this consumption of the costly diphenyl is not economical. If, on the other hand, there is to little diphenyl present, the pyrolysis produces this substance, to the detriment of the yield of terphenyls. This shows the advantage of using the mixtures in amounts of diphenyl as near as possible to that which does not vary during the course of the pyrolysis; in this manner of carrying out the process, the terphenyls are produced solely from benzene and there is neither production nor consumption of diphenyl.

Another important factor in the process is the length of time of the pyrolysis, that is to say, the residence period, the influence of which on the yield is evident from the graphs described above. The process can be carried out with residence periods of from 5 to seconds, while the preferred periods are of the order of 10 to 50 seconds.

This factor, as has been seen, is related to others, that is to say, temperature and proportion of diphenyl. If, for a given proportion of diphenyl, the temperature is too low or the residence period too short, terphenyls are produced from the diphenyl present, which is thus consumed. On the other hand, if, for the given amount of diphenyl, the temperature is too high or the residence period too long, part of the benzene is converted to diphenyl and is thus consumed other than for the production of terphenyls. The process, according to the invention, can provide a proportion of conversion of benzene into terphenyls of 5% to 18% with respective yields of 75% to 60%, according to the duration of the residence period and, consequently, the proportions of diphenyl in the pyrolysis vapor.

The advantages of the invention are illustrated by means of ten examples, the numerical results of which are set out in the form of a table below. For each example, the reaction conditions and results are set out as follows:

The temperature of the vapor is given for the point of entry (A) to the pyrolysis tube and the point of exit (B) from the tube; the arithmetic mean (C) of these two temperatures is also given.

Table of Examples Example No 1 2 3 4 5 6 7 8 9 10 REACTION CONDITIONS Temperature C 0.):

At, entry to zone (A) 650 700 700 660 680 680 800 800 At exit from zone (B) 750 760 790 790 850 755 790 790 820 820 Mean -0 (C) 700 730 745 745 800 707 735 735 810 810 Residence period in zone (R) 5 12 10 15 15 50 20 40 3 5 Composition of pyrolysis, mixture (percent by wt.):

Benzene E 70 65 80 78 73 72 80 Diphenyl (F)..- 100 30 35 20 20 22 27 28 20 RESULTS Proportions of mixture consumed:

Benzene (G) 3. 5 12. 6 27. 5 9. 5 11.5 21 15. 3 16. 4 Diphenyl "(H)" 12 20 14 g xmo -.0)" 4. 4 8.7 12 3. 4

mifil xmo (10-- s 14 5. 4 5.75 11.3 6.5 7. 5 12.6 1.8 6.95

Yield 0f terpheny1s 50 0 7r 0 5 12 4 per consumption (G and/ or H) 7 a 70 70 60 The residence period (R) is, as indicated above, the time taken for the vapors to traverse the pyrolysis zone, and thus it is the eifective period of heating.

The composition of the vapor treated, that is to say, of the pyrolysis mixture, is expressed as percent by Weight of benzene (E) and of diphenyl (F) in the mixture.

The consumption of benzene (G) and of diphenyl (H) are shown separately, which allows it to be seen under what conditions the benzene alone forms the starting ma terial for the production of the terphenyls. These figures of consumption are percent by weight of the amount of compound originally introduced.

On the other hand, the production of diphenyl (J) represents the rate of transformation of benzene into diphenyl when the latter is formed in addition to terphenyls. The figure for the terphenyls formed (K) indicate the percentage of terphenyls obtained with respect to the weight of the material from which they were formed, that is to say, with respect to diphenyl alone (Examples 1 and 2), benzene alone (Examples 4 to or both (Example 3). This figure naturally relates to a single passage of vapor down the heated tube Without recycling.

The yield of terphenyls (Y) is calculated on the amounts of star-ting material(s) actually consumed; it is thus the quotient of the amount of terphenyls formed (K) divided by the consumption of benzene (G), of diphenyl (H), or of both, multiplied by 100.

Referring to the data in the table, Example 1 is a comparative example of the preparation of terphenyls from diphenyl alone without any benzene; it is given for comparison with Examples 2 to 8 which are examples according to the invention of preparation from mixtures of benzene and diphenyl. Such comparison shows that there is a marked improvement in the results when mixtures are employed in place of diphenyl alone.

In Example 1 and 2 the terphenyls are produced solely from the diphenyl; in Example 1, where no benzene at all is present, 6% of terphenyl are obtained with a yield of 50% on the diphenyl; in Example 2 these figures are respectively 14% and 70%, thanks to the presence of a strong proportion of benzene in the pyrolysis mixtures.

It is of interest to note from Example 9 that at the temperatures and times there chosen, 35% of diphenyl (F) in the initial mixture is too high a proportion. Indeed, in these conditions, 14% of diphenyl (H) is transformed into terphenyls against 3.5% only of benzene (G) converted.

By contrast, in Examples 4 and 5 the amount of of diphenyl (F) is shown to be too low since the pyrolysis produces respectively 4.4 and 8.7% (I) of undesired diphenyl.

Examples 6, 7 and 8 show the preferred manner of carrying out the invention, especially from the economic point of view; it is seen that there is, indeed, a constancy of amount of diphenyl (F)--22%, 27%, 28%-which is neither produced nor consumed during the pyrolysis.

The terphenyls are thus formed only from the benzene, an inexpensive starting material; the proportion of terphenyls produced (K) is from 6.5% to 12.6% and the yields (Y) from to 60%. The amounts can furthermore be as high as 18% and the yields as high as under such conditions.

Example 9 is a comparative example, the inverse of comparative Example 1, showing the results of using benzene vapor alone without any diphenyl, and also the effect of using a very short pyrolysis time, 3 seconds; the yield of terphenyls (Y) is very low, 11.9%.

Example 10 illustrates the eifects of using a short residence period, 5 seconds, and a high temperature, 800 to 820 C.; the yield of terphenyls (Y) is 42.5%, which is much better than Example 9 but poorer than that of Examples 2 to 8 according to the invention.

While certain novel features of the .invention have been shown and described and are pointed out in the annexed claim, it will be understood that various omissions, substitutions and changes in the forms and details of the process illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention.

Invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

A process for the production of terphenyls by the pyrolysis of benzene in the gaseous phase, comprising the steps of heating a mixture consisting of diphenyl vapor and benzene vapor and free from sulfur to a temperature between 600 C. and 750 C., said mixture containing diphenyl in the range from 20% to 30% by weight and containing benzene in the range from to 70% by weight; admitting said mixture to a pyrolysis zone wherein it is maintained at a temperature in the range from 680 C. to 820 C. for a period of time in the range of from 10 to 50 seconds; the mixture entering the pyrolysis zone being at a temperature in the range from 600 C. to 700 C. and the products leaving the pyrolysis zone being at a temperature in the range from 750 C. to 820 C.; condensing at least a portion of the products leaving the pyrolysis zone to the liquid state; and separating the terphenyls from the liquid so obtained, said process being characterized in that the proportion by weight of diphenyl in the gaseous mixture admitted to the pyrolysis zone is equal to the proportion by weight of diphenyl in the mixture leaving the pyrolysis zone, so that there is no net consumption of diphenyl in said process, and this proportion remains substantially unchanged during the pyrolysis.

DELBERT E. GANTZ, Primary Examiner. ALPHONSO D. SULLIVAN, Examiner. 

